The invention relates more particularly to using a time reversal technique to transmit a signal between two communicating entities including at least one transmit antenna and at least two receive antennas.
An antenna signal is a radio signal transmitted by an antenna of a communicating entity and is subject to distortion as a function of the propagation conditions between a source point defined at the output of the source antenna and a destination point defined at the input of an antenna of the destination communicating entity. To limit this distortion, the antenna signal is predistorted by applying pre-equalization coefficients as a function of the characteristics of the propagation channel between these two antennas. It is therefore necessary to characterize this propagation channel.
Of existing pre-equalization methods, methods using time reversal are distinguished by their reduced complexity, their high performance and their intrinsic capacity for focusing a radio wave onto a receive antenna. Time reversal makes it possible to reduce significantly the time dispersion of the propagation channel by focusing the energy of the received signal in time and in space.
Time reversal is a technique for focusing waves, typically acoustic waves, that relies on the invariance of the wave equation on time reversal. Thus a time-reversed wave propagates like a forward wave traveling back in time. A short pulse emitted from a source point propagates in a propagation medium. Part of this wave received by a destination point is time reversed before it is sent back in the propagation medium. The wave sent back converges toward the source point, where it forms a short pulse, and the energy of the wave is focused on the source point (focus). The shape of the signal focused on the source point by time reversal is virtually identical to that of the source signal emitted at the source point. Thus time recompression occurs at the focus. In particular, the more complex the propagation medium, the more precisely the time-reversed wave converges.
The time-reversal technique is therefore applied in radio communications networks to cancel the effect of the propagation channel on the antenna signal, notably by reducing the spreading of the channel, and to simplify the processing of symbols received after passing through the channel. The antenna signal emitted by an antenna of the source communicating entity is thus pre-equalized by application of coefficients obtained by time reversing the impulse response of the propagation channel that this antenna signal has to pass through. The time reversal of the propagation channel applied to the signal makes it possible to cancel the effect of this channel on transmission from the source point of the signal predistorted in this way and to focus the signal on a destination antenna. Time reversal therefore requires knowledge of the propagation channel by the source communicating entity.
In bidirectional time-division duplex (TDD) transmission, transmission in a first direction, for example from a source communicating entity to a destination communicating entity, and transmission in a second direction that is the opposite of the first direction are effected on the same carrier frequency and at different times. The propagation channel corresponding to the first direction is then substantially identical to the propagation channel corresponding to the second direction. A source communicating entity is therefore capable of estimating the propagation channel from the received signals.
In frequency-division duplex (FDD) bidirectional transmission, transmission in the first direction and transmission in the opposite direction are effected in different frequency bands. The source communicating entity's knowledge of the propagation channel corresponding to the first transmission direction may be obtained from an estimate of the propagation channel produced by the destination communicating entity, for example.
However, if the communicating entities are mobile, the channel estimate produced at a given time by a communicating entity may prove erroneous afterwards because of the mobility of the communicating entities. This error is qualified relative to a defined relative movement as a function of the movement of the source communicating entity as observed from the destination communicating entity. For large relative movements there is a decorrelation between the estimate of the propagation channel used to predistort the signal and the propagation channel that the signal actually experiences.
Accordingly, firstly the predistortion of the signal is inadequate and secondly the signal is not focused onto a destination antenna. The time-reversal pre-equalization technique therefore offers very poor performance for fast-moving communicating entities.
The paper entitled “From Theory to Practice: an overview of MIMO space-time coded wireless systems”, by David Gesbert, Mansoor Shafi, Da-shan Shui, Peter J. Smith, and Ayman Naguid, published in the IEEE Journal on Selected Areas in Communication, Vol. 21, April 2003 expresses the effect of the relative movement as a function of wavelength, the propagation channel measurement period and delays between the measurement and the reception of the transmitted antenna signal. That paper then shows that the time-reversal technique can be used only for short propagation channel measurement periods and very short delays between the measurement and the reception of a signal pre-equalized as a function of the measurement.
One solution is to estimate the relative movement or to estimate the movement of the source communicating entity in order to use the time-reversal method or not. Thus European Patent application EP 0 109 533, entitled “A method and device for channel estimation in a mobile system, particularly in a mobile phone”, proposes a channel estimation method including a step of estimating the movement of a mobile terminal. Depending on the estimated movement, the most suitable transmission technique is adopted, for example time reversal for a small movement and a transmission technique that does not require channel estimation for large movements.
That solution thus requires the use of two transmission modes in a communicating entity. Apart from the increase in complexity, the choice of the movement threshold or the relative movement enabling selection of the appropriate transmission mode is difficult because it is a function of the propagation context.
There is therefore a need for a method of dynamically compensating the movement of a source communicating entity relative to a destination communicating entity for transmitting a signal based on a time-reversal technique that is effective for a wide range of relative movements of the communicating entities. Below, the expression relative movement must be understood as referring to the movement of the source communicating entity as observed from the destination communicating entity.